proto/third_party: Check in submodules for protobuf and googleapis dependencies.

Update Makefile and README for dependencies.

.gitmodules

@@ -0,0 +1,6 @@
+[submodule "proto/third_party/protobuf"]
+	path = proto/third_party/protobuf
+	url = git@github.com:protocolbuffers/protobuf.git
+[submodule "proto/third_party/googleapis"]
+	path = proto/third_party/googleapis
+	url = git@github.com:googleapis/googleapis.git

proto/Makefile

@@ -1,4 +1,4 @@
-THIRD_PARTY_ROOT := ../../proto/third_party
+THIRD_PARTY_ROOT := ./third_party
 
 GOOGLEAPIS := $(THIRD_PARTY_ROOT)/googleapis
 PROTOBUF := $(THIRD_PARTY_ROOT)/protobuf/src

proto/README.md

@@ -1,27 +1,16 @@
-The top-level directory for protobuf message and service definitions.
+Protobuf message and service definitions.
 
 `Makefile` generates golang message and service stub implementations into
 //go/gen/proto/. The generated code is checked into the repository.
 
-Typescript and javascript artifact generation has been disabled for the time being.
-
 Dependencies
 ------------
 
-You need `protoc-gen-go` and `ts-protoc-gen`. To build `ts-protoc-gen`, you
-need to install bazel.
+Dependencies are git submodules in //proto/third_party. You need to build
+protoc in //proto/third_party/protobuf by running `bazel build //:protoc` from
+that directory. Currently tested with bazel version 0.28.0.
 
-```
-$ go get -u github.com/golang/protobuf/protoc-gen-go
-
-$ mkdir -p $HOME/src/3p
-$ pushd $HOME/src/3p
-$ [ -d ts-protoc-gen ] || git clone git@github.com:improbable-eng/ts-protoc-gen.git
-$ cd ts-protoc-gen
-$ git checkout 0.7.6
-$ bazel build //...
-$ popd
-```
+Dependency tracking and hermeticity here poor.
 
 Generating code
 ---------------